JP2015034324A - Steel excellent in rolling fatigue life - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel for a machine component which regulates the contents of oxygen, sulfur and Al in the steel, the average composition ratio of an MgO-AlObased oxide and the number ratio of the MgO-AlObased oxide in the total oxides, and has excellent Llife and rolling fatigue life.SOLUTION: In steel excellent in rolling fatigue life, the content of oxygen is 8 ppm or less; the content of sulfur is 0.008% or less; the content of Al is 0.005 to 0.030%; the number of non-metallic inclusions detected in the steel material per 1000 mmwith ultrasonic flaw detection, having the inclusion diameter of 20 μm or more and less than 100 μm, is 12.0 pieces or less; the number of non-metallic inclusions detected in the steel material per 2.5 kg with ultrasonic flaw detection, having the inclusion diameter of 100 μm or more, is 2.0 pieces or less; the mass% ratio MgO/AlOin an MgO-AlObased oxide present in the steel is 0.25 to 1.50; and the number ratio of the MgO-AlObased oxide in the total oxide based inclusions is 70% or more.

Description

  The present invention relates to a mechanical component used for curing a surface hardness of 58 HRC or more, such as a bearing, a gear, a hub unit, a toroidal type CVT device, a constant velocity joint, a crankpin, etc. The present invention relates to steel applied as a device.

  In recent years, due to the improvement in performance of various mechanical devices, the use environment of mechanical parts and devices that require a rolling fatigue life has become severe. Along with this, there are increasing demands for improving the life and reliability of these components and devices. In response to such demands, measures for steel surfaces include optimization of steel components and reduction of impurity elements that are harmful to rolling fatigue life, thereby improving life and improving reliability.

  Among the impurity elements contained in the steel composition, for example, oxygen is an element that constitutes oxide inclusions that can be a starting point of damage such as alumina. Therefore, especially oxygen with high toxicity has been reduced to the ppm order. When higher quality is required, the oxygen amount may be further reduced by special dissolution such as VAR and ESR. In addition, measures are taken to prevent adverse effects of other impurity elements by reducing their content to the order of 0.01% by mass.

By the way, various high cleanliness steels with a small amount of oxygen in the steel have been proposed. Among these proposals, regarding the number of oxides in steel, a high carbon-based long life with a value of {(MgO · Al ₂ O ₃ number + MgO number) / total oxide-based inclusions} is 0.80 or more. Bearing steel has been proposed (see, for example, Patent Document 1). In Patent Document 1, the composition range of MgO and Al ₂ O ₃ is not particularly shown, and the display is not MgO—Al ₂ O ₃ but MgO · Al indicating that it is a stoichiometric composition. _Since it is expressed by molecular formula as ₂ O ₃ , it is shown as a compound composed of 28.3% MgO and 71.7% Al ₂ O ₃ by mass%. Furthermore, a high carbon chromium bearing steel in which the total number of alumina and spinel is less than 60% of the total number of oxides and a method for producing the same have been proposed (for example, see Patent Document 2). As far as this patent document 2 is concerned, alumina is less than 3% of (MgO) and (SiO ₂ ), and (CaO) is a ratio of (CaO) / ((CaO) + (Al ₂ O ₃ )). The spinel system is a binary system (MgO) in the range of 3% to 20% and the balance being (Al ₂ O ₃ ), and within 15% of (CaO) and / or Alternatively, it is defined as having a spinel crystal structure in which up to 15% of (SiO ₂ ) may be mixed. Further, for high cleanliness bearings, the oxygen content in the steel is less than 10 ppm, and the exposed surface area of oxide inclusions floated and aggregated by the electron beam melting method is 20 μm ² or less per gram. Steel has been proposed (see, for example, Patent Document 3). On the other hand, the steel with excellent rolling fatigue life targeted by the present invention, that is, the L ₁ life in the thrust type rolling fatigue test (99% of the tests when the same lot of test pieces are tested under the same conditions) The occurrence of non-metallic inclusions exceeding 20 μm, which affects the L ₁ life, is extremely accidental and low in stably providing a steel having an excellent cycle number that can be rotated without peeling. Since it occurs with probability, it is very difficult to detect these occurrences, and in the steel described in Patent Document 3, inclusions are melted and agglomerated, so the accurate inclusion diameter and number should be evaluated. I can't. Further, in the conventional method for evaluating non-metallic inclusions, it is impossible to judge whether the steel material is good or bad because it takes a long time to inspect a large volume of the steel material because the test area is small.

Both methods, such as applying an extreme value statistical method to inclusions having a maximum inclusion diameter of about 100 μm or less, and applying an ultrasonic flaw detection method with a flaw detection frequency of 5 to 25 MHz for inclusions of about 100 μm or more. There has been proposed an evaluation method using the above (see, for example, Patent Document 4). In this method, the extreme value statistical method is applied to nonmetallic inclusions having a maximum inclusion diameter of less than 100 μm, and the flaw detection frequency is set to 5 to 25 MHz for nonmetallic inclusions having a diameter of 100 μm or more. We have proposed an evaluation method using a combination of methods such as applying the law. However, it is difficult to say that the extreme value statistical method can sufficiently judge the quality of a steel material when the non-metallic inclusions having a small test area as described above and having a size of 20 μm or more and less than 100 μm are viewed. On the other hand, since the inclusion diameter detected by the ultrasonic flaw detection method with a flaw detection frequency of 5 to 25 MHz is 100 μm or more, it is still impossible to sufficiently evaluate inclusions of 20 μm or more and less than 100 μm. , hard to say that the evaluation method that can provide an excellent steel L ₁ life stability. In addition, steels that define the number and size of inclusions as steels with excellent rolling fatigue life have been proposed by evaluating the inclusions of 100 μm or less by ultrasonic flaw detection with a flaw detection frequency of 20 to 125 MHz. (For example, see Patent Document 5). By the way, in the method described in Patent Document 5, a non-metal having a sulfur content of 0.008% by mass or less and an inclusion diameter detected per 300 mm ³ of steel material volume by an ultrasonic flaw detection method is 20 μm or more. Steel with excellent rolling fatigue life, specified to have 12 or less inclusions per 300 mm ³ (in a thrust type rolling fatigue test, maximum hertz stress P _max = 5.3 GPa and L ₁₀ life> 1 Steel with which 0.0 × 10 ⁷ cycle can be obtained) and its evaluation method. However, since the reliability of the bearings in use being damaged very early than the calculated life has not been evaluated, the L ₁ life (a test piece of the same lot was tested under the same conditions) as a measure of reliability for early damage. In some cases, 99% of the specimens are not steels that can stably provide a steel having an excellent cycle number that rotates without peeling.

JP-A-8-3682 JP 2006-200027 A JP-A-6-192790 JP 2006-317192 A JP 2008-121035 A

An object of the present invention is to suppress damage that is extremely early with respect to the calculated life in a machine part that requires a rolling fatigue life. Therefore, the inventors used the L ₁ life as a measure of reliability (that is, when the same lot of test pieces are tested under the same conditions, 99% of the test pieces rotate without peeling). I paid attention to. This L ₁ life has not been evaluated at all by the prior art.

Therefore, the inventors have conducted intensive studies on the control of non-metallic inclusions for improving rolling fatigue life, and in particular, on means for reducing the influence of oxide-based non-metallic inclusions that are highly harmful to rolling fatigue life. did. As a result, the hard oxide inclusions in steel, which had been necessary to be avoided in the prior art, are appropriate for those containing Al ₂ O ₃ and MgO in the composition ratio and number ratio. It has been found that the L ₁ life can be improved by further modifying the number of non-metallic inclusions in the steel per certain amount by the ultrasonic flaw detection method.

That is, in order to make the steel excellent in L ₁ life capable of suppressing the very early peeling with respect to the calculated life, particularly for the parts requiring rolling fatigue life, the oxygen content in the steel is 8 ppm or less by mass ratio, Sulfur content is 0.008% by mass or less, Al content is 0.005 to 0.030% by mass, and non-metallic inclusions are detected per 1000 mm ³ volume of steel by ultrasonic flaw detection. The diameter of the object (hereinafter referred to as “inclusion diameter”) is 20 μm or more and less than 100 μm, and the number of non-metallic inclusions is 12.0 or less. The average of MgO—Al ₂ O ₃ -based oxides present in steel, with the inclusion diameter detected per 5 kg, the inclusion diameter is 100 μm or more, the number of nonmetallic inclusions is 2.0 or less (MgO) / (in composition The weight percent ratio of l ₂ O _3), 0.25~1.50, more preferably restricted to a range of 0.30 to 1.30, and total oxide system MgO-Al ₂ O ₃ based oxide It has been found that the number ratio of inclusions may be regulated to 70% or more, preferably 80% or more. The MgO—Al ₂ O ₃ -based non-metallic inclusions defined herein may include those containing 15% or less of CaO by mass% and / or 15% or less of SiO ₂ by mass%. The reason why the oxygen content is 8 ppm or less by mass and the sulfur content is 0.008 mass% or less is the size and frequency of the presence of oxide inclusions and sulfide inclusions that are relatively soft and easy to stretch. This is to reduce the above. More preferably, the oxygen content is 6 ppm or less by mass and the sulfur content is 0.003 mass% or less. Furthermore, in order not to modify the soft inclusions, and to suppress the formation of pure alumina (Al ₂ O ₃ ) which is hard but tends to agglomerate and form clusters in the steel, the Al content is 0.005 to 0.005. It is necessary to set it as 0.030 mass%, More preferably, it is 0.008-0.030 mass%, More preferably, it is 0.011-0.030 mass%.

In steels in which the average composition of oxide inclusions is regulated and the number ratio of oxide inclusions to all oxide inclusions is restricted to the above 70% or more, preferably 80% or more, oxidation is not possible. Since the material has a composition having a high melting point, a small-diameter oxide crystallizes in a nearly spherical shape from the molten steel in the process of producing a steel ingot. In this way, even if it is crystallized in a nearly spherical shape, pure alumina (Al ₂ O ₃ ) that tends to agglomerate in the molten steel afterwards is suppressed, so oxidation in the ingot after the molten steel solidifies. The physical inclusions are dispersed in a small diameter and nearly spherical shape.

  Furthermore, if the ingot is rolled into a steel bar by hot working, and then the steel bar is used as a raw material, it will be oxidized into a steel bar or steel pipe as a component material or a forged product by further hot working or cold working. Since inclusions are harder than the parent phase steel in the hot or cold processing temperature range, they are less likely to deform following the parent phase during processing, and therefore remain relatively spherical after processing. A close shape can be maintained.

  After that, the steel bars and pipes used as component materials are subjected to further cold processing such as CRF, if necessary, and then are processed by cutting, and further, the surface desired for the components subjected to rolling fatigue by appropriate heat treatment. After adjusting the hardness to 58HRC or more, it is used as a machine part, but the maximum stress acting direction under the transfer surface of the part subjected to rolling fatigue is the minimum cross section of the non-metallic inclusions in the steel material used as the material of the part. For example, in oxide inclusions and sulfide inclusions that are relatively soft and stretched by hot working, the direction perpendicular to the rolling direction may not always match.

Therefore, the inventors experimentally melted steel containing oxide inclusions that are relatively soft at high temperatures and stretched by hot working, and using the hot rolled steel of the steel as a raw material, Thrust-type rolling fatigue life test is performed using the surface that coincides with the rolling direction, which is the maximum cross-sectional direction of oxide inclusions, as a transfer surface, and the L ₁ life is evaluated as a reliability index for debonding with an extremely short life. As a result, it has been found that the L ₁ life is reduced as compared with the case where the direction perpendicular to the rolling direction is the transfer surface. This is because inclusions having a soft oxide composition at a high temperature have a low melting point, so the frequency of occurrence is rare, but large inclusions remain in the steel, and the heat of the inclusions. L ₁₀ life (test specimen of the same lot), which is estimated to be because the direction of the maximum cross-section after hot rolling (that is, the size of the defect) is almost coincident with the direction of maximum stress action, and is evaluated as an index of normal part life When the test is performed under the same conditions, 90% of the test pieces are less likely to appear in the number of cycles that rotate without peeling), but this is clarified by the evaluation of the L ₁ life. Similarly to oxide inclusions with a composition that tends to soften hot, the difference in maximum cross-section size of inclusions in the rolling direction and in the direction perpendicular to the rolling direction is due to the extension of inclusions during processing. Therefore, as described above, the L ₁ life may be inferior depending on the way of taking the transfer surface of the part.

In contrast, the oxygen content forming oxides in the steel proposed by the inventors and the sulfur content forming sulfides are both reduced, and the oxide inclusions in the steel have a small diameter. Further, in the steel dispersed in a shape close to a spherical shape, unlike the above results, the L ₁ life in the thrust type rolling fatigue life test in which the surface coincident with the rolling direction is the transfer surface is improved. This is the headline and the present invention. In other words, the size and frequency of oxides and sulfides in the steel used as the component material are sufficiently reduced, and oxide inclusions in steel that are particularly harmful to rolling fatigue life are reduced in diameter. In addition, by dispersing in a nearly spherical shape, the maximum stress acting direction in rolling fatigue is always maintained regardless of the direction in which the transfer surface when processed into a part is arranged with respect to the rolling direction or stretching direction of the original material. Since the inclusion cross-sectional area with respect to can be minimized, the harmfulness to rolling fatigue is reduced, and the rolling fatigue life is improved. In addition, the present invention provides an index of separation with an extremely short life by appropriately regulating the number of non-metallic inclusions contained in steel per fixed amount by ultrasonic flaw detection. ₁ Steel with excellent life can be obtained stably.

In order to solve the problem to be solved by the present invention, all the steels described in Patent Documents 1 to 5 have not been evaluated for the L ₁ life, and have reliability against extremely early peeling with respect to the calculated life of the component. Not guaranteed steel. In the steel described in Patent Document 1, the value of {(MgO · Al ₂ O ₃ + MgO number) / total number of oxide inclusions} is regulated to 0.80 or more with respect to the number of oxides in the steel. However, it is an essential condition to modify the oxide composition to be mainly oxide having a stoichiometric composition of MgO.Al ₂ O ₃ or MgO. For this purpose, Mg addition in the refining process and Mg in the steel Since the inclusion is essential, the manufacturing cost is increased and the versatility is poor. Further, oxygen content and not be said also sufficient for regulation of sulfur content, since the content frequently non-metallic inclusions in the steel be not evaluated, stable technology that can provide a steel excellent in L ₁ life is not.

Further, in the steel described in Cited Document 2, the total number of alumina (Al ₂ O ₃ main) and spinel (MgO—Al ₂ O ₃ ) is regulated to be less than 60% of the total number of oxides. by performing the softening control of composition of inclusions and, while thereby improving the L ₁₀ life, the present invention is MgO-Al ₂ O ₃ system total number of oxide at least 70% of the total oxide quantity The L ₁ life as an index of reliability against peeling at an extremely short life is improved by regulating so that the technical idea is completely different.

  In addition, Patent Documents 3 to 5 do not provide any suggestion regarding modification of the chemical composition or the number ratio of hard oxide inclusions in steel. Moreover, in the steel of patent document 3, there are few steel samples for evaluating the surface exposure area of oxide inclusions at about 1-5 g, and melting and aggregation of inclusions occur by the electron beam melting method. Therefore, it cannot be said that it is sufficient as an index for evaluating the cleanliness of steel per a certain amount necessary for improving the reliability with respect to peeling with an extremely short life, which is the object of the present invention.

Moreover, in the steel of patent document 4, since sufficient evaluation about the nonmetallic inclusion which is 20 micrometers or more and less than 100 micrometers cannot be performed, in the steel of patent document 5, the diameter of the inclusion The number of non-metallic inclusions having a thickness of 20 μm or more is specified to be 12 or less per 300 mm ³ , but the regulation is not sufficient in light of the present invention, so that both are L ₁ lifetimes It is hard to say that it is a method that can stably provide excellent steel.

The present invention was made to solve such conventional problems, and the problems to be solved by the present invention are to regulate the oxygen content, sulfur content, and Al content in steel, weight% ratio of the average composition of MgO-Al ₂ O ₃ based oxide _{(MgO) / (Al 2 O} 3), MgO-Al 2 O 3 based oxide number ratio of total oxides, as well as in steel By regulating the number of non-metallic inclusions of 20 μm or more per fixed quantity and less than 100 μm and the number of non-metallic inclusions of 100 μm or more per fixed quantity in steel, L ₁ which is an index of very early peeling. The purpose is to provide a steel for machine parts having an excellent rolling fatigue life by improving the life.

Means for solving the above-mentioned problem relates to steel used in a machine part having a surface hardness of 58 HRC or more in the first means. That is, the oxygen content in the steel of this steel is 8 ppm or less, the sulfur content is 0.008 mass% or less, the Al content is 0.005 to 0.030 mass%, and an ultrasonic flaw detection method. The number of inclusions (hereinafter referred to as “inclusion diameter”) detected per volume of 1000 mm ³ of the steel material is 20 μm or more and less than 100 μm, and the number of nonmetallic inclusions is 12.0 or less. . Furthermore, the number of non-metallic inclusions with a inclusion diameter of 100 μm or more, which is detected per 2.5 kg of the weight of the steel material by an ultrasonic flaw detection method, is 2.0 or less, and The mass% ratio of (MgO) / (Al ₂ O ₃ ) in the average composition of the MgO—Al ₂ O ₃ oxide present is regulated to a range of 0.25 to 1.50, and MgO—Al ₂ O comprising a number ratio of total oxide inclusions of ₃ based oxide because it was 70% or more, an excellent steel rolling fatigue life.

The second means relates to steel used for machine parts having a surface hardness of 58 HRC or more. That is, the oxygen content in the steel of this steel is 6 ppm or less, the sulfur content is 0.003 mass% or less, the Al content is 0.005 to 0.030 mass%, and an ultrasonic flaw detection method. Thus, the number of inclusions with a inclusion diameter of 20 μm or more and less than 100 μm detected per 1000 mm ³ of the steel material is 9.0 or less. Furthermore, by ultrasonic testing, the number of inclusions having a diameter of 100 μm or more, which is detected per 2.5 kg of the steel material, is 1.5 or less, and in the steel The mass% ratio of (MgO) / (Al ₂ O ₃ ) in the average composition of the MgO—Al ₂ O ₃ oxide present is regulated to a range of 0.25 to 1.50, and MgO—Al ₂ O comprising a number ratio of total oxide inclusions of ₃ based oxide because it was 70% or more, an excellent steel rolling fatigue life.

In the third means, the number of inclusions with non-metallic inclusions having an inclusion diameter of 20 μm or more and less than 100 μm was evaluated by flaw detection with a total volume of 1500 mm ³ or more by ultrasonic flaw detection, and the inclusion The number of non-metallic inclusions having an object diameter of 100 μm or more was evaluated by flaw detection of a total weight of 3.0 kg or more by ultrasonic flaw detection, and the rolling fatigue of the first means or the second means. Steel with excellent life.

In the fourth means, the steel having excellent rolling fatigue life is high carbon chromium bearing steel (SUJ) specified in JIS standard, 52100 specified in SAE standard or ASTM standard A295, and 100Cr6 specified in DIN standard. As well as any one of the steel materials for machine structural carbon steel (SC) and alloy steel materials for mechanical structure defined in JIS standards. The alloy steel material for machine structure specified in this JIS standard is any one steel selected from chromium steel (SCr), chromium molybdenum steel (SCM), or nickel chromium molybdenum steel (SNCM). The steel is excellent in rolling fatigue life of any one of the first to third means.
Further, the present invention can be applied to foreign standard steels corresponding to JIS standards such as SAE standards 4320, 5120, 4140, 1053, and 1055.

In the steel excellent in rolling fatigue life of the present invention, the oxygen content, sulfur content, and Al content in the steel are regulated, and (MgO) in the average composition of MgO—Al ₂ O ₃ oxides in the steel /% Ratio of (Al ₂ O ₃ ) and the number ratio of MgO-Al ₂ O ₃ -based oxides to the total oxides, and the non-metallic inclusions in steel are greatly increased by ultrasonic flaw detection. By using a steel with a limited number of non-metallic inclusions detected by volume, it is possible to obtain a steel used for machine parts having an excellent rolling fatigue life.

  The steel excellent in rolling fatigue life according to the embodiment of the present invention will be described in detail below with reference to the table.

In the embodiment of the invention of claim 1, a steel excellent in rolling fatigue life according to an embodiment of the present invention is a steel used for a machine part having a surface hardness of 58 HRC or more. The oxygen content is 8 ppm or less, the sulfur content is 0.008 mass% or less, and the Al content is 0.005 to 0.030 mass%. Furthermore, the number of inclusions having a inclusion diameter of 20 μm or more and less than 100 μm detected per steel material volume of 1000 mm ³ by an ultrasonic flaw detection method of 25 to 125 MHz is 12.0 or less. Furthermore, the number of inclusions with a inclusion diameter of 100 μm or more, which is detected per 2.5 kg of steel material by an ultrasonic flaw detection method of 5 to 25 MHz, is 2.0 or less. Furthermore, the mass% ratio of (MgO) / (Al ₂ O ₃ ) in the average composition of MgO—Al ₂ O ₃ -based oxides present in this steel is regulated to a range of 0.25 to 1.50, and , A steel with excellent rolling fatigue life in which the number ratio of MgO—Al ₂ O ₃ oxide to the total oxide inclusions is 70% or more.

In an embodiment of the invention of claim 2, the steel is used for a machine part having a surface hardness of 58 HRC or more, and the oxygen content in the steel is 6 ppm or less by mass and the sulfur content is 0.00. It is 003 mass% or less, and Al content is 0.005-0.030 mass%. Furthermore, the number of inclusions with a inclusion diameter of 20 μm or more and less than 100 μm, which is detected per 1000 mm ^{3 of} steel material by an ultrasonic flaw detection method of 25 to 125 MHz, is 9.0 or less. Furthermore, the number of non-metallic inclusions with an inclusion diameter of 100 μm or more, which is detected per 2.5 kg of steel material weight by an ultrasonic flaw detection method of 5 to 25 MHz, is 1.5 or less. Furthermore, the mass% ratio of (MgO) / (Al ₂ O ₃ ) in the average composition of MgO—Al ₂ O ₃ -based oxides present in this steel is regulated to a range of 0.25 to 1.50, and , A steel with excellent rolling fatigue life in which the number ratio of MgO—Al ₂ O ₃ oxide to the total oxide inclusions is 70% or more.

In an embodiment of the invention of claim 3, the number of non-metallic inclusions having an inclusion diameter of 20 μm or more and less than 100 μm is evaluated by flawing a total volume of 1500 mm ³ or more by an ultrasonic flaw detection method of 25 to 125 MHz. It has been done. Further, the number of non-metallic inclusions having an inclusion diameter of 100 μm or more was evaluated by flaw detection with a total weight of 3.0 kg or more by an ultrasonic flaw detection method of 5 to 25 MHz. Steel with excellent life.

  In the embodiment of the invention of claim 4, it is desirable that the steel excellent in rolling fatigue life is a steel type used for applications requiring rolling fatigue life including bearings. Specifically, high carbon chromium bearing steel (SUJ) specified in JIS standard, 52100 specified in SAE standard or ASTM standard A295, 100Cr6 specified in DIN standard, for machine structure specified in JIS standard Examples of the steel material include any one of carbon steel materials and alloy steel materials for machine structural use. As an alloy steel material for machine structure defined in this JIS standard, any one steel selected from chromium steel (SCr), chromium molybdenum steel (SCM), or nickel chromium molybdenum steel (SNCM) therein Further, for example, the present invention can be applied to foreign standard steels corresponding to JIS standards such as SAE standards 4320, 5120, 4140, 1053, and 1055. This steel is excellent in rolling fatigue life in item 1.

  In the ultrasonic flaw detection method described above, various types of ultrasonic flaw detectors and probes are already on the market, and these can be used. As a preferable probe, a focus type high frequency probe and the like can be cited. The detection capability of the flat probe is said to be ½ wavelength, but the focus probe is ¼ wavelength, and the focus probe is suitable for accurate evaluation. For inclusions having an inclusion diameter of 20 μm or more and less than 100 μm in the present embodiment, the probe frequency is preferably about 25 to 125 MHz, particularly preferably about 30 to 100 MHz. In addition, for inclusions having an inclusion diameter of 100 μm or more in this embodiment, the probe frequency is preferably about 5 to 25 MHz, and these are already as in the embodiment of claim 3 above.

In ultrasonic flaw detection, the total volume for confirming the number of inclusions for inclusions having an inclusion diameter of 20 μm or more and less than 100 μm is set to 1500 mm ³ or more, and the number of inclusions is confirmed for inclusions having an inclusion diameter of 100 μm or more. The reason why the total weight is 3.0 kg or more is that it is necessary to obtain a satisfactory evaluation result in terms of evaluation accuracy in providing steel that can provide a stable rolling fatigue life. In addition, the evaluation volume and the evaluation weight in the ultrasonic flaw detection method according to the present embodiment cannot be evaluated practically because the processing time is enormous in the conventional evaluation method mainly based on microscopic observation. It is. When performing ultrasonic flaw detection, the dead zone area from the surface of the specimen to the depth corresponding to the probe frequency is excluded from the evaluation volume, and if necessary, tissue abnormalities due to heat treatment etc. and measurement noise in ultrasonic flaw detection are detected. After excluding the end of the sensitive specimen from the ultrasonic beam flaw detection range at the focal position, the evaluation volume for ultrasonic flaw detection is determined based on the underwater focal length range according to the probe frequency and performance. 1500 mm ³ or more (when confirming the number of inclusions with an inclusion diameter of 20 μm or more and less than 100 μm) and an evaluation weight in ultrasonic flaw detection of 3.0 kg or more (confirming the number of inclusions with an inclusion diameter of 100 μm or more) Need to ensure).

The melting of the mother molten steel of the present invention may be performed by either the electric furnace method or the blast furnace-converter method. Subsequently, the mass% ratio of (MgO) / (Al ₂ O ₃ ) in the average composition of the MgO—Al ₂ O ₃ oxide in steel and the number ratio of the MgO—Al ₂ O ₃ oxide are evaluated. The method will be described below.

In the steel having an excellent rolling fatigue life of the present embodiment, the (MgO) / (Al ₂ O ₃ ) mass% ratio in the average composition of the MgO—Al ₂ O ₃ -based oxide, and the MgO—Al ₂ O ₃ Energy dispersive X-ray analysis of oxide inclusions with an inclusion diameter of 1 μm or more in a test area of at least 40 mm ² or more selected from any part of the steel cross section in order to accurately evaluate the number ratio of system oxides Thus, the component analysis of the oxide composition and the count of the number of oxides are performed. Based on the oxide count and its composition analysis result, it may be calculated number ratio of the average composition, and MgO-Al ₂ O ₃ based oxide MgO-Al ₂ O ₃ system oxides in the steel. Note that the oxide complexed with sulfides and nitrides for elements constituting the sulfides and nitrides, and shall determine the average composition of MgO-Al ₂ O ₃ based oxide was excluded for the element .

As described above, according to the steel having an excellent rolling fatigue life according to the present embodiment, the oxygen content, the sulfur content, and the Al content in the steel are regulated, and an ultrasonic flaw detection method is used in the steel. Non-metallic inclusions are detected in a large volume, and the average composition of MgO—Al ₂ O ₃ oxides in steel and the number ratio of MgO—Al ₂ O ₃ oxides to the total oxides are regulated. Thus, it is possible to provide steel used for machine parts having excellent rolling fatigue life.

  Next, the steels excellent in rolling fatigue life of the present invention will be described more specifically by taking the test materials 1 to 28 as examples and the test materials 29 to 34 as comparative examples. However, the present invention is not limited to these examples.

  Table 1 shows the component composition of the test materials. JIS SUJ2 steel, which is a high carbon chromium bearing steel, is used for the test materials 1 to 10 and 29 to 32 in Table 1, and 52100 defined in the SAE standard is used for the test materials 11 and 12. The test material 13 and the test material 14 are 52100 defined in ASTM standard A295, the test material 15 and the test material 16 are 100Cr6 defined in the DIN standard, and the test material 17 is JIS SUJ3 steel, JIS SUJ5 steel for specimen 18, JIS SCr420 steel for specimen 19 and specimen 33, SAE 5120 steel for specimen 20, specimen 21 and For specimen 34, JIS SCM420 steel, for specimen 22, JIS SNCM420 steel, for specimen 23, SA 4320 steel, JIS SCM435 steel for specimen 24, SAE 4140 steel for specimen 25, JIS S53C steel for specimen 26, and JIS for 27 specimen. The S55C steel was used, and the sample material 28 was SAE 1053 steel. Specimens 1 to 34 were melted in an arc melting furnace, subsequently smelted in a ladle, and further degassed with a vacuum degasser to produce an ingot by continuous casting.

At that time, for the test materials 1 to 28 of the examples, an appropriate oxide composition is prepared by appropriately adjusting the slag composition while appropriately collecting the sample in advance in the refining process of the molten steel and confirming the inclusion composition. After studying to satisfy the range and the number ratio, the mother molten steel was melted. On the other hand, for the test material 29 and the test material 30 of the comparative example, the addition of Al to the molten steel is suppressed during the refining process of the mother molten steel, and Si deoxidation is mainly performed to improve the soft inclusions. Done quality. Furthermore, test pieces 31-34 of the comparative example has less MgO-Al ₂ O ₃ based oxide by performing intentionally added to deoxidation of Al in the molten steel in the refining process of the mother molten steel, the Al ₂ O ₃ Modification was performed so that the main oxide was obtained.

(Thrust type rolling fatigue test)
The steel materials of the test materials 1 to 18 and the test materials 29 to 32 were subjected to spheroidizing annealing at 800 ° C., the outer diameter was 52 mm from the direction parallel to the longitudinal direction of the steel material, the inner diameter was 20 mm, and the thickness was 5.8 mm. A test piece made of a disk was prepared. After holding this test piece at 835 ° C. for 20 minutes, it was quenched by oil cooling and then subjected to tempering treatment at 170 ° C. for 90 minutes to obtain a desired hardness of 58 HRC or more, followed by surface polishing and thrust. A mold rolling fatigue test was conducted. The steel materials of the test materials 19 to 23, the test material 33 and the test material 34 were normalized at 925 ° C., and the steel materials of the test material 24 and the test material 25 were 870 ° C. After normalizing, a test piece made of a disk having an outer diameter of 52 mm, an inner diameter of 20 mm, and a thickness of 8.3 mm from a direction parallel to the longitudinal direction of the steel material was produced. This test piece was carburized at 930 ° C., then quenched by oil cooling, then tempered at 180 ° C. for 90 minutes to obtain a desired hardness of 58 HRC or higher, and then surface polished to obtain a thrust type A rolling fatigue test was conducted. The steel materials of the test materials 26 to 28 were normalized at 870 ° C., and a test piece consisting of a disk having an outer diameter of 52 mm, an inner diameter of 20 mm, and a thickness of 8.3 mm from a direction parallel to the longitudinal direction of the steel material was produced. did. This test piece was induction hardened, and then tempered at 180 ° C. for 90 minutes to obtain a desired hardness of 58 HRC or higher, and then subjected to surface polishing to perform a thrust type rolling fatigue test. The thrust type rolling fatigue test was performed at a maximum Hertz stress Pmax: 5.3 GPa. In obtaining the L ₁ life, the test evaluation time was shortened by a censoring test at about 1.5 × 10 ⁷ cycles.

(Evaluation of oxide composition and number ratio)
The mass% ratio of (MgO) / (Al ₂ O ₃ ) in the average composition of MgO—Al ₂ O ₃ -based oxides present in the steel is 0.25 to 1.50, and MgO—Al ₂ O ₃ In evaluating that the number ratio of the system oxide to the total oxide inclusions is 70% or more, the steel materials of the test materials 1 to 18 and the test materials 29 to 32 were subjected to spheroidizing annealing at 800 ° C. After application, the steel materials of the test materials 19 to 23, the test material 33, and the test material 34 were normalized at 925 ° C., and the steel materials of the test materials 24 to 28 were 870 ° C. After normalizing, a test piece having a thickness of 7 mm and a test area of 100 mm ² of 10 mm in the longitudinal direction and 10 mm in the radial direction is cut out from a direction parallel to the longitudinal direction of the steel material, and is non-metallic during polishing. In order to prevent the inclusions from falling off, the surfaces to be inspected are mirror-polished after quenching and tempering. Subjecting were counted oxide number and component analysis of the oxide composition by energy dispersive X-ray analysis. Based on the composition analysis result and the oxide count, the (MgO) / (Al ₂ O ₃ ) mass% ratio in the average composition of the MgO—Al ₂ O ₃ -based oxide in the steel, and MgO—Al ₂ O ₃ The number ratio of the system oxide was calculated.

About each test piece of these test materials, surface hardness, mass% ratio of (MgO) / (Al ₂ O ₃ ) in the average composition of MgO—Al ₂ O ₃ -based oxide in steel, and MgO—Al Table 2 shows the number ratio of ₂ O ₃ -based oxides.

In Table 2, the specimens 29 to 34 of the comparative examples are (MgO) / (Al ₂ O ₃ ) mass% ratio in the average composition of MgO—Al ₂ O ₃ -based oxide in steel and / or steel. The number ratio of the number of MgO—Al ₂ O ₃ based oxides is outside the scope of the present invention. With respect to the test materials 29 to 34 of these comparative materials, the mass% ratio of (MgO) / (Al ₂ O ₃ ) in the average composition of MgO—Al ₂ O ₃ -based oxide in steel, and MgO— in steel All of the number ratios of the Al ₂ O ₃ -based oxides are excellent in L ₁ life, as will be described later, as compared with the comparative examples. ing.

(Ultrasonic test)
In evaluating non-metallic inclusions whose inclusion diameter is 20 μm or more and less than 100 μm, the steel materials of specimens 1 to 18 and specimens 29 to 32 are subjected to spheroidizing annealing at 800 ° C. After cutting out and quenching and tempering, the specimens 19 to 23, specimen 33, and specimen 34 were subjected to normalization at 925 ° C., and L-section specimens were cut out. After quenching and tempering, the steel materials 24 to 28 were normalized at 870 ° C., L-section test pieces were cut out, quenched and tempered, and then subjected to ultrasonic transmission loss. In order to alleviate this, surface polishing was performed. Each surface was finished to a thickness of 10 mm by surface polishing, and an ultrasonic flaw detection test was conducted. For ultrasonic flaw detection, an ultrasonic flaw detector equipped with a focus type high-frequency probe (50 MHz) was used. The ultrasonic flaw detection volume was 3000 mm ³ . The number of detected inclusions of 20 μm or more and less than 100 μm per 1000 mm ³ of the volume of the steel material was determined from the data of the reflected wave by the obtained inclusions.

  Moreover, when evaluating the nonmetallic inclusion whose inclusion diameter is 100 μm or more, the steel materials of the test materials 1 to 18 and the test materials 29 to 32 were subjected to spheroidizing annealing at 800 ° C., and an L cross section test. After cutting out the pieces, the steel materials of the specimens 19 to 23, the specimen 33, and the specimen 34 were normalized at 925 ° C., and the L-section specimen was cut out, and then the specimens 24 to 24 were cut out. About 28 steel materials, after normalizing at 870 degreeC and cutting out the L cross-section test piece, all performed the surface grinding | polishing and finished in thickness 45mm, and the ultrasonic flaw test was done. For ultrasonic flaw detection, an ultrasonic flaw detector equipped with a focus type high-frequency probe (10 MHz) was used. The ultrasonic flaw detection weight was 10.0 kg. The number of detected inclusions of 100 μm or more per 2.5 kg of the weight of the steel material was determined from the data of the reflected wave by the obtained inclusions.

About each test piece of these test materials, surface hardness, the number of detected inclusions per 1000 mm ³ of steel volume by ultrasonic flaw evaluation evaluated with a 50 MHz focal high frequency probe, 10 MHz focal high frequency probe Table 3 shows the number of inclusions detected per 2.5 kg of steel weight by ultrasonic flaw evaluation and the L ₁ life measured by the thrust type rolling fatigue test.

In Table 3, sample materials 1-5, sample material 11, sample material 13, sample material 15, sample material 18-20, sample material 25-27 of the examples satisfy the present invention. Yes, even if the L ₁ life (relative value based on the comparative example 32) is the lowest, it is 3.3 of the specimen 1.

In this case, the oxygen content in the steel is 8 ppm or less in mass ratio, the sulfur content is 0.008 mass% or less, and the inclusion diameter detected per 1000 mm ³ of steel volume by the ultrasonic flaw detection method is 20 μm or more. The number of non-metallic inclusions less than 100 μm is 12.0 or less, and the number of non-metallic inclusions detected with a inclusion diameter of 100 μm or more per 2.5 kg of steel weight is 2.0. And the mass% ratio of (MgO) / (Al ₂ O ₃ ) in the average composition of MgO—Al ₂ O ₃ -based oxides present in the steel is in the range of 0.25 to 1.50. In addition, the present invention satisfies the inventions of claim 1 and claim 3 in which the number ratio of the MgO—Al ₂ O ₃ oxide to the total oxide inclusions is 70% or more.

Further, the oxygen content in the steel is 6 ppm or less by mass ratio, the sulfur content is 0.003 mass% or less, and the inclusion diameter detected per 1000 mm ³ of the steel material by the ultrasonic flaw detection method is 20 μm or more. The number of non-metallic inclusions of less than 100 μm is 9.0 or less, and the number of non-metallic inclusions detected per 2.5 kg of steel material weight is 100 μm or more is 1.5. And the mass% ratio of (MgO) / (Al ₂ O ₃ ) in the average composition of MgO—Al ₂ O ₃ -based oxides present in the steel is in the range of 0.25 to 1.50. And, the number ratio of the MgO-Al ₂ O ₃ based oxide to the total oxide inclusions is 70% or more, the test materials 6 to 10, the test material 12, the test material 14, and the like, The test material 16, the test material 17, the test materials 21 to 24, and the test material 28 are Is intended to satisfy the light of the invention of claim 2 and claim 3, (relative value relative to the Comparative Example 32) L ₁ life was 4.3 test material 12 be of a minimum, the rolling fatigue The steel has a better life.

In contrast, in the test materials 29 to 34 of the comparative example, the number of non-metallic inclusions that are 20 μm or more and less than 100 μm detected per 1000 mm ³ of the steel material volume exceeds 12.0, and the steel material weight is 2. The number of non-metallic inclusions of 100 μm or more detected per 5 kg exceeds 2.0 and (MgO in the average composition of MgO—Al ₂ O ₃ -based oxides present in the steel) ) / (Al ₂ O ₃ ) mass% ratio is outside the range of 0.25 to 1.50, and the number ratio of the MgO—Al ₂ O ₃ oxide to the total oxide inclusions is 70%. It is outside the scope of the present invention. The test materials 29 to 34 of these comparative examples are inferior to the test material 31 of 2.2 and this example even though the L ₁ life (relative value based on the comparative example 32) is the maximum. ing.

Claims (4)

Steel used for machine parts having a surface hardness of 58 HRC or more, wherein the oxygen content in the steel is 8 ppm or less by mass, the sulfur content is 0.008 mass% or less, and the Al content is 0.005 to 0. 0.030% by mass, and the number of non-metallic inclusions having an inclusion diameter of 20 μm or more and less than 100 μm detected per 1000 mm ³ of steel by ultrasonic flaw detection is 12.0 or less, and The number of non-metallic inclusions with an inclusion diameter of 100 μm or more detected per 2.5 kg of steel material by ultrasonic flaw detection is 2.0 or less, and MgO—Al ₂ O present in the steel. in the average composition of ₃ based oxide mass% ratio of _{(MgO) / (Al 2 O} 3) was restricted to a range of 0.25 to 1.50, and total oxidation of MgO-Al ₂ O ₃ based oxide The ratio of the number of physical inclusions to 70% or more Steel excellent in rolling fatigue life and wherein the a. Steel used for machine parts having a surface hardness of 58 HRC or more, wherein the oxygen content in the steel is 6 ppm or less by mass, the sulfur content is 0.003 mass% or less, and the Al content is 0.005 to 0. 0.030% by mass, and the number of non-metallic inclusions having an inclusion diameter of 20 μm or more and less than 100 μm detected per 1000 mm ³ of steel by ultrasonic flaw detection is 9.0 or less, and The number of non-metallic inclusions with an inclusion diameter of 100 μm or more detected per 2.5 kg of steel material by ultrasonic flaw detection is 1.5 or less, and MgO—Al ₂ O present in the steel. in the average composition of ₃ based oxide mass% ratio of _{(MgO) / (Al 2 O} 3) was restricted to a range of 0.25 to 1.50, and total oxidation of MgO-Al ₂ O ₃ based oxide The ratio of the number of physical inclusions to 70% or more Steel excellent in rolling fatigue life and wherein the. The number of non-metallic inclusions having an inclusion diameter of 20 μm or more and less than 100 μm was evaluated by flaw detection of a total volume of 1500 mm ³ or more by the ultrasonic flaw detection method, and the inclusion diameter was 100 μm or more. The number of certain nonmetallic inclusions was evaluated by flaw detection with a total weight of 3.0 kg or more by ultrasonic flaw detection, and excellent in rolling fatigue life according to claim 1 or 2. steel.   Steels with excellent rolling fatigue life are specified in high carbon chromium bearing steels specified in JIS standards, 52100 specified in SAE standards or ASTM standards A295, and 100Cr6 specified in DIN standards, and in JIS standards. A carbon steel material for machine structure or an alloy steel material for machine structure is any one steel selected from chrome steel, chrome molybdenum steel, and nickel chrome molybdenum steel. The steel excellent in the rolling fatigue life of any one of these.